An Introduction to Solar System Astronomy
Prof. Scott Gaudi
Lecture 25: Measuring Light:
Every atom, ion, and molecule has a unique spectral
- Reflection of their the underlying electron orbital
Absorption and Emission of Photons
- Excitation and De-Excitation
- Remove one or more electrons, or add an extra electron.
Looking inside the Atom
Electrons cannot orbit just anywhere around a nucleus:
The details are dictated by quantum mechanics.
- Can only orbit in discrete orbitals.
- Each orbital corresponds to a particular energy of the
- If an electron does not have exactly the right energy, it
cannot be in that orbital (all or nothing).
Hydrogen: The Simplest Atom
An atom of Hydrogen (1H) consists of:
- A single proton in the nucleus.
- A single electron orbiting the nucleus.
First orbital: Ground State (n=1)
- Lowest energy orbital the electron can reside in.
Higher orbitals: Excited States (n=2,3,...)
- Higher orbits around the nucleus.
- Come at specific, exact energies.
Emission & Absorption Lines
When an electron jumps from a higher to a lower energy
orbital, a single photon is emitted with exactly the
energy difference between orbitals. No more, no less.
Electrons can get into the excited states by either
- Colliding with other atoms or free electrons
- Absorbing photons of specific energies...
When an electron absorbs a photon with exactly the
energy needed to jump from a lower to a higher orbital.
Absorption is very specific:
The excited states decay by emitting photons in random directions.
- Only photons with the exact excitation energy are absorbed.
- All others pass through unabsorbed.
If an atom or molecule absorbs enough energy from a photon or
a collision, an electron can be ejected.
Similarly, you can also add extra electrons:
- Get a Positive Ion (atom or molecule with a
net positive charge).
Ions differ from their parent neutral atoms or molecules:
- Get a Negative Ion (atom or molecule with a net negative charge).
- Diferent spectral line signatures.
- Different chemical properties.
Other atoms have more electrons, and hence more complex
electron orbital structures.
- Results in more complex line spectra.
- There is a unique spectrum for each element, reflecting
its unique electron orbital structure.
- Isotopes show the same lines, but slightly shifted in
Every element has its own, distinctive spectral signature.
Molecules are more complex still:
- Compounds of two or more atoms, of the same or different elements.
- Share some electrons in common orbitals.
Results in very complex spectra:
Molecules mainly produce strong lines at infrared, microwave, and radio
- Broad "bands" consisting of many lines.
- Bands often span large wavelength regions.
The Importance of Spectroscopy
From the emission or absorption lines in an object's spectrum,
we can learn:
These data give us a nearly complete picture of the physical
conditions in the object.
- Which atoms and molecules are present, and in what proportions.
- Which atoms are ionized, and in what proportions.
- How excited (or not) the atoms are, tells us the objects state
(e.g., hot or cold).
Spectroscopy is one of the most important tools of the astronomer.
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